There is a need for so-called “single photon sources” for use in optical quantum cryptography where, for example, the security key for an encryption algorithm is formed by sending a stream of single photons which are regularly spaced in time. It is essential for the security of this technique that each bit is encoded on just a single photon. This is because an eavesdropper trying to intercept the communication will be forced to measure and thereby alter the state of some photons. Therefore, the sender and the intended recipient can determine if their communication has been intercepted.
Such a source is also useful as a low-noise source for optical imaging, spectroscopy, laser ranging and metrology. Normal light sources suffer from random fluctuations in the photon emission rate at low intensities due to shot noise. This noise limits the sensitivity of many optical techniques where single photons are detected. A single photon source which produces photons at regular time intervals has a reduced shot noise.
Recently, advances have been made in making such single photon sources from semiconductor quantum dot structures. Michler et al in “A Quantum Dot Single-Photon Turnstile Device” Science 290 p 2282 to 2284 (2000) and Santori et al “Triggered Single Photons from a Quantum Dot” Physics Review Letters 86 p 1502 to 1505 (2001) also describe single photon sources which operate by optically pumping a single quantum dot.
The above devices all concentrate on the production of a photon from the decay of a single neutral exciton (simple exciton) where prior to emission of the single photon there is a single electron in the ground state of the conduction band of the quantum dot and a single hole in the valence band of the quantum dot.
However, photons can also arise from the decay of more exotic excitons such as charged excitons or higher order excitons such as bi-excitons, triexcitons, quad excitons etc. Warburton et al, Nature 405 p926 to 929 describes photons arising from neutral and single, double, triple, quadruple and quintuple negatively charged single excitons in a quantum ring.
Findeis et al, Phys Rev B 63 p 121309-1 to 121309-4 (2001) and Finley et al, Phys Rev B 63 073307-1 to 073307-4 (2001) report observations of charged excitons from quantum dots.
Finally, Hartmann et al, Phys Rev Lett 84 p 5648 to 5651 describes experiments where they observe the presence of bi-excitons and higher order excitons in quantum dots.
In every single photon source where a photon is emitted due to recombination of an electron and a hole, there is always some uncertainty in the actual time when the photon will be emitted. This is usually referred to as the “jitter”. The applicants have surprisingly found that by using photons resulting from the decay of higher order excitons or charged excitons, the jitter of the photon source can be reduced.